In the field of sporting goods, especially in the field of winter sporting goods, there is a continued desire to improve the mechanical properties of the sporting gear in order to allow for a safer, more customized and faster sporting experience.
In particular, when manufacturing winter sport footwear, such as for example skiing boots or skating boots, a key property is the flex modulus of the material employed in the outer shell of the boot. Boots that are too stiff or rigid are not well received among athletes, because they are uncomfortable and unresponsive. Expressed alternatively, the flex modulus of the boot material needs to be reasonably low over a wide range of temperatures of from −20° C. to 20° C.
For this reason, the currently used materials in boots are “softer” polymers such as for example ionomers or thermoplastic polyurethanes (TPUs), which have a comparatively low flex modulus and yield flexible and responsive skiing boots.
However, while boots made of ionomer have excellent flex properties at temperatures of form about −20° C. to about 20° C., they have a propensity to deform under creep stress. For example, in a car stationed in the sunlight, temperature can easily climb to 50 or 70° C., a temperature at which the currently used ionomers starts deforming or warping. This is of course undesirable, because after cooling down, the boot will not fit in the intended way onto the foot of the wearer. Repeated heating and cooling further exacerbate this problem.
While not suffering from deformation under creep stress, boots made from thermoplastic polyurethanes have other drawbacks. For instance, the flex properties of TPUs can change dramatically over the ranges encountered in winter that is of from about −20° C. to about 20° C. While the flex properties of TPUs are satisfactory in the upper parts of the range, TPUs rigidify significantly (by up to 400%) in the lower parts of the range, i.e., the colder it becomes, the less flexible and comfortable boots made of TPU become.
In recent years, there have been efforts to customize skiing boots by adapting the foam liners of the boot to the feet of the wearer upon purchase by using heat curable foams. This method however, was limited in that the polymeric outer shell of the boot was not adapted to the anatomy of the wearer, because of the temperatures necessary to soften the polymeric shell.
Polymers such as TPUs or polyamides require very high temperatures of up to 200° C. to reach a point where they are soft enough, which makes it impossible to adjust the boot, because the user is required to shoe the boot during the adjustment process.
On the other hand, ionomers may be suitable for boot shells that can be adjusted at temperatures around 50° C., but they suffer from the above-mentioned deformation problems of creep deformation.
There is therefore a strongly felt need for a polymeric composition exhibiting suitable flex properties, even across a broad temperature range, that can be used for sporting footwear, which does not suffer from the above-mentioned deformation problems under heat stress and that can be adjusted to the anatomy of the wearer without exposing him to a burning risk.